1. Field of the Invention
The present invention relates generally to an apparatus for recording and reproducing video signals, which includes a preemphasis means for modifying video signals on time base prior to recording into the recording medium and a deemphasis means for modifying the video signals in a complementary manner, thereby reducing the effects of noise introduced in the system.
2. Description of the Prior Art
In the recording of video signals into the recording medium such a magnetic tape and the reproduction of the same, there is known that reducing the effects of noise is a big issue. A method having been conventionally used to encounter this problem is the combination of a preemphasis process wherein the magnitude of high frequency components is increased with respect to the magnitude of others and a deemphasis process wherein the magnitude of high frequency components is decreased with respect to the magnitude of others in a complementary manner. With this method, an S/N ratio of the video signal can be improved. For example, Japanese Patent No. 3-9679/1991 discloses a recording and reproducing apparatus carrying out such preemphasis and deemphasis processes.
FIG. 20 is a circuit block diagram showing such a conventional video recording and reproducing apparatus. In the drawing, an input video signal Vi arriving from an input terminal 1 is supplied to a preemphasis circuit 2. The preemphasis circuit 2 modifies the input video signal Vi on time base through a preemphasis process wherein the magnitude of high frequency components is increased with respect to the magnitude of low frequency components. The video signal Vi is then supplied into a record signal processing circuit 3, in which the video signal Vi is, for example, frequency modulated into a form suitable for the recording. The video signal Vi, thus modulated in the record signal processing circuit 3, is supplied to a magnetic head (not shown but generally well known) through an appropriate amplifier (not shown). The magnetic head interacts with a magnetic tape 4 to magnetically record the video signal Vi into the magnetic tape 4. The recording operation is completed in this manner.
In the reproduction of video signals from the magnetic tape 4, a video signal Vo recorded in the magnetic tape 4 is read out through the magnetic head and the amplifier. A reproduction signal processing circuit 5 receives this output video signal Vo and demodulates it. Subsequently, the video signal Vo is supplied into a deemphasis circuit 6. The deemphasis circuit 6 modifies the video signal Vo through a deemphasis process wherein the magnitude of high frequency components is decreased with respect to the magnitude of the low frequency components. The deemphasis circuit 6 and the preemphasis circuit 2 are complementary with each other. The video signal Vo is then transmitted to other components from an output terminal 7.
FIG. 21 shows a circuit configuration of the preemphasis circuit 2. In the drawing, the input video signal Vi arriving an input terminal 10 is supplied to both an adder circuit 11 and a subtracter circuit 12. The video signal Vi outputted from the adder circuit 11 is subsequently supplied into a delay circuit 13. The delay circuit 13 has a delay time corresponding to 2m fields or {(2m+1) fields-0.5H}, where m=0, 1, 2, . . . and H being one horizontal period.
The delayed video signal Vi, generated from the delay circuit 13, is then supplied into a coefficient circuit 14, in which the delayed video signal Vi is multiplied by a predetermined coefficient K1 to reduce its magnitude. The delayed video signal Vi is thereafter fed back from the coefficient circuit 14 to the adder circuit 11 and added with a succeeding video signal Vi supplied from the input terminal 10. An output signal, i.e. a result of the addition in the adder circuit 11, is then supplied into the delay circuit 13.
The delayed video signal Vi, generated from the delay circuit 13, is meanwhile supplied into a coefficient circuit 15, in which the delayed video signal Vi is multiplied by a predetermined another coefficient K2. The delayed video signal Vi is thereafter supplied into the subtracter circuit 12 and subtracted from the succeeding video signal Vi supplied from the input terminal 10. The delayed video signal Vi, obtained from the coefficient circuit 15, corresponds to video signals inputted in advance by a plurality of predetermined periods equivalent to one field and its magnification. As a result of this subtraction, the subtracter 12 produces a video signal Vi having a preemphasis characteristic in which the magnitude of high frequency components is enhanced on time base with respect to the magnitude of low frequency components. The video signal Vi is thereafter transmitted to the record signal processing circuit 3 through an output terminal 16.
FIG. 22 shows a circuit configuration of the deemphasis circuit 6. The output video signal Vo, supplied from the reproduction signal processing circuit 5 preceding the deemphasis circuit 6, is taken in through an input terminal 20. The output video signal Vo, having the preemphasis characteristic, is supplied to adder circuits 21 and 22. The video signal Vo outputted from the adder circuit 21 is subsequently supplied into a delay circuit 23. The delay circuit 23 has the same delay time as that of the delay circuit 13 of FIG. 21.
The delayed video signal Vo, generated from the delay circuit 23, is then supplied into a coefficient circuit 24, in which the delayed video signal Vo is multiplied by a predetermined coefficient N1. The delayed video signal Vo is thereafter fed back from the coefficient circuit 24 to the adder circuit 21 and added with a succeeding video signal Vo arriving from the input terminal 20. An output signal, i.e. a result of the addition in the adder circuit 21, is then supplied into the delay circuit 23.
The delayed video signal Vo, generated from the delay circuit 23, is meanwhile supplied into a coefficient circuit 25, in which the delayed video signal Vo is multiplied by a predetermined another coefficient N2. The delayed video signal Vo is thereafter supplied into the adder circuit 22 and added with the succeeding video signal Vo supplied from the input terminal 20. As a result of this addition, the adder 22 produces the video signal Vo having a deemphasis characteristic in which the magnitude of high frequency components is reduced on time base with respect to the magnitude of low frequency components.
The deemphasis characteristic obtained in this deemphasis circuit 6 is complementary with the preemphasis characteristic obtained in the preemphasis circuit 2. This means that the deemphasis circuit 6 can reconstruct the video signal into the original form. Namely, the video signal Vo produced from the adder 22 is equivalent in its waveform with the input video signal Vi arriving the input terminal 10 of the preemphasis circuit 2.
FIGS. 23 and 24 show another examples of the preemphasis circuit and deemphasis circuit, respectively. Components denoted by the same reference numerals as those of FIG. 21 and 22 show like parts and, therefore, will no more be explained. These examples are equivalent to the previous example in that they can provide preemphasis and deemphasis characteristics similar to those of the previous examples.
Furthermore, in editing video tape, electronic editing has been conventionally used in the field of VCR related technologies. The electronic editing is classified into several modes, one of which is called as an assemble mode. This assemble mode will be explained hereinafter with reference to FIGS. 25(A)-25(D).
As shown in the drawings, this assembly mode is roughly characterized in that necessary portions of programs are successively recorded on a master tape T. While a start timing of recording is strictly determined, an end timing of recording is not specially designated in this editing mode. If a sufficient time has passed to record a necessary portion, the recording is ended at an appropriate timing as shown in FIG. 25(A). After the editing of the first cut shown in FIG. 25(A), the master tape T is rewound by an amount d as shown in FIG. 25(B). This rewind amount d should be identical with an unnecessary portion recorded at the end region of the first cut. Then, a second cut is edited from a start timing newly set in this manner, as shown in FIG. 25(B). FIGS. 25(c) and 25(D) which respectively show third and fourth cuts being edited in the same manner.
FIG. 26 shows the mutual relationship, i.e. recording sequence, between an original tape TO and the master tape T. A point 0-IN represents a playback start point from which the original tape TO starts playback. Likewise a point M-IN represents a recording start point from which the master tape T begins recording. After these points O-IN and M-IN have been both specified, these original and master tapes T0 and T are rewound a little bit by the same amount p. This rewind motion is referred to as "pre-roll". Next, both tapes T0 and T initiate travelling simultaneously in playback mode. As soon as the both tapes T0, T reach the specified points O-IN and M-IN, the master tape T is switched into the record mode to start recording from this point M-IN.
Although the combination of above-described preemphasis and deemphasis processes associated in a complementary manner brings an excellent improvement of S/N ratio, this combination on the other hand causes a problem when the assemble mode electronic editing is carried out. In more detail, each of the preemphasis and deemphasis circuits 2, 6 of FIG. 20 constitutes a circular type noise reduction circuit. This circuitry feature, although advantageous in the noise reduction, becomes a direct cause of undesirable residual image or other adverse effects when video signals are recorded together with another video signals in an overlapped manner according to the assemble mode editing explained with reference to FIGS. 25(A)-25(D).
Let us suppose that a certain region of the magnetic tape 4 already stores video signals recorded by the use of preemphasis process. And, another video signals are newly recorded from a point on or before the end of this already recorded region. As the previous video signals are still circulated in a feedback loop consisting of the delay circuit 13, coefficient circuit 14, and adder circuit 11 in the beginning of the recording of newly recorded video signals, the newly recorded video images are applied the preemphasis process on the basis of those circulated previous video images and, subsequently, recorded on the magnetic tape 4.
In reproducing video signals newly recorded in this manner, the deemphasis circuit 6 still circulates the previous video signals in its feedback loop and, therefore, carries out the deemphasis process on the basis of the previous video signals.
This discrepancy of video signals to be processed in the preemphasis and deemphasis circuits, occurring temporarily in the switching stage of video signals from the previous ones to the present ones, becomes a great cause of residual images and other adverse effects.
The inconvenience of residual images and other adverse effects derived from the circular nature of the preemphasis and deemphasis circuits would be also serious in the case where the previously recorded region succeeds the end of the newly recorded region because similar discrepancy occurs in the preemphasis and deemphasis circuits in the switching stage of video signals.